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According to this link, "The wavelength at which the $O_2$ molecule most strongly absorbs light is approximately $145$ nm."

According to this link, that's in the ultraviolet range of the electromagnetic spectrum.

Consider two tanks containing oxygen gas, both equivalent. One tank has a steady stream of $145$ nm ultraviolet light being emitted on it, while the other has a flame warming the bottom of the tank.

The temperature of the oxygen in both tanks will increase. Say we calibrate the experiment so both tanks reach $10^{\circ}$ C (and that nothing explodes)

My question is, is there any difference in the oxygen contained in one tank vs. the other? In other words, is there any experiment that could be done to determine whether some oxygen came from one tank or the other?

Or, since they are at the same temperature, are they equivalent?

The question arises because from what I understand, a gas molecule heats up by radiative absorption by absorbing a photon, which excites the electrons in it, while the mechanism by which it is heated via conduction/convection is different (molecules bumping into each other ?), and I'm not sure if this results in a different "quality" of heated gas.

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  • $\begingroup$ The configuration of two tanks is unclear. Also, are you asking about heating mechanism or about resulting thermodynamic state - which is independent on the mechanism? $\endgroup$
    – Roger V.
    Mar 10, 2023 at 17:00
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    $\begingroup$ According to this site skydayproject.com/…. Very high energy UV rays (with a wavelength shorter than 240 nm) are absorbed by molecular oxygen, causing the molecule to split into two individual oxygen atoms. From there, an individual oxygen atom can react with molecular oxygen to form ozone. $\endgroup$
    – Bob D
    Mar 10, 2023 at 17:09

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As noted in @Bob_D's comment, 145 nm radiation is going to convert many oxygen (O$_2$) molecules into ozone (O$_3$). Ozone is unstable and eventually reverts to molecular oxygen, but its half-life is long enough that it could be detected for quite some time.

The half-life of ozone in a glass vessel initially filled with 10% O$_3$ and 90% O$_2$at at ambient temperature and pressure is about 20 hours. The lifetime is less is some other types of vessels, e.g. it is only about an hour in brass. Ambient surface ozone levels are $<10^{-7}$, so an oxygen tank initially filled with 10% ozone might be distinguishable for about 20 half-lives, i.e. a couple of weeks.

Of course, figuring out how long your two tanks would be distinguishable is not so simple. Even if you specified many more details, it is not trivial to estimate the initial ozone concentrations and their decay. A 172 nm lamp in dry oxygen at 300K can produce 400 g/kWh of ozone (assuming 100% wall plug efficiency). The specific heat of oxygen is 0.92 J/gK, so a crude dimensional estimate is that we might expect the ozone concentration to be $\sim 10^{-4}K^{-1} \Delta T$. Increasing the temperature of oxygen by $10$K with a UV lamp should roughly produce $\sim 0.1\%$ ozone concentration, which could be distinguishable for a week or so.

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When you heat a gas by electromagnetic radiation with the molecules absorbing the incident radiation, what happens is that energy which at first is internal to each molecule then gets shared among molecules by collisions, so what what was an internal excitation energy gets transferred into other forms of energy such as motion of the molecule as a whole. All this happens on a timescale of nanoseconds at ordinary pressure and temperature. The molecules will also emit much of the radiation so overall the heating of the gas is a result of an average over many such processes. Since each process is so quick the gas overall heats up in a way quite like the way it does when heated by some other mechanism such as a flame warming the bottom of the tank. So the answer to your question is that the two processes leave the gas in the same state.

There is one detail however. When heating by absorbing radiation, while the radiation is there some of the atoms or molecules are in an excited electronic state and this is different from the other case. But as soon as the radiation is switched off those atoms will decay to their ground state very rapidly (in microseconds) and after that there is no significant difference between the two gases.

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    $\begingroup$ So theoretically one could distinguish between a gas that is being heated by absorbtive radiation from one that is being heated by conduction if one could detect the excited states. Of course, one could also measure the temperature gradients and deduce heating by conduction, but that would be boring? $\endgroup$
    – TimWescott
    Mar 10, 2023 at 21:13
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    $\begingroup$ Yes. To detect excitation you could look at emitted light using a spectrometer, or look for a change in the absorption. $\endgroup$ Mar 11, 2023 at 14:42

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